This application is related to the following co-pending applications, all filed on the same day and naming the same inventors as this application: xe2x80x9cProcessing a Spread Spectrum Signal in a Frequency Adjustable Systemxe2x80x9d; xe2x80x9cBandpass Processing of a Spread Spectrum Signalxe2x80x9d; xe2x80x9cAcquiring a Spread Spectrum Signalxe2x80x9d; and xe2x80x9cReceiving a Spread Spectrum Signalxe2x80x9d.
The invention-relates to processing a spread spectrum signal.
In wireless systems, information typically is transmitted by modulating the information onto carrier waves having frequencies that lie within preassigned frequency bands. Radio frequency (RF) receivers demodulate the carrier waves to recover the transmitted information.
Spread spectrum communication systems spread transmitted signals over bandwidths much larger than those actually required to transmit the information. Spreading a signal over a wide spectrum has several advantages, including reducing the effects of narrow band noise on the signal and, in many situations, providing increased protection against interception by unwanted third parties. In a direct sequence spread spectrum (DSSS) system, the bandwidth of a transmitted signal is increased by modulating the signal onto a known pseudo-noise (PN) signal before modulating onto the carrier wave. The PN signal typically is a digital signal having an approximately equal number of high and low bits (or xe2x80x9cchipsxe2x80x9d), which maximizes the spectrum over which the signal is spread. A typical implementation of a DSSS receiver recovers the transmitted information by demodulating the carrier wave and then multiplying the resulting signal with a local replica of the PN signal to eliminate the PN signal. The DSSS technique offers heightened security because the receiver must know the PN sequence used in the transmission to recover the transmitted information efficiently. Other spread spectrum techniques include frequency hopped spread spectrum (FHSS).
In one aspect, the invention features processing a spread spectrum signal digitally sampled at a selected sampling rate and tuned to a center frequency lower than the sampling rate. The digitally sampled spread spectrum signal is multiplied against a reference signal to produce a corresponding product output, and an accumulation output is formed by combining the product output with a phase-shifted version of the accumulation output.
Embodiments of the invention may include one or more of the following features. The center frequency may be set to equal approximately one-quarter the sampling rate. The accumulation output may be phase-shifted by 180xc2x0 or 360xc2x0 before it is combined with the product output. A delay element may be used to shift the accumulation output by two or four samples to produce the 180xc2x0 or 360xc2x0 phase-shifted version of the accumulation output, which may be accomplished using two or four, respectively, single-sample delay registers.
Advantages of the invention may include one or more of the following. A spread spectrum signal may be processed without demodulating the signal to DC until the processing is complete. An AC-coupled filter or other DC block may be used to eliminate DC offset from the spread spectrum signal before it is processed. A direct sequence spread spectrum signal may be processed using a correlator with an implementation efficient structure. For example, if the spread spectrum signal is processed at a center frequency equal to one-quarter the digital sampling rate (Fs/4), the signal may be processed by a correlator structure that produces an accumulation output simply by subtracting from the output of a multiplier a 180xc2x0 phase-shifted version of the accumulation output formed by delaying the accumulation output by two samples. Such a correlator may be constructed using very few and inexpensive components, which will lower the cost of DSSS receivers using the structure. Such a correlator structure also will improve the capabilities and efficiency of such receivers.
Other advantages of the invention will become apparent from the following description and from the claims.